Speech transmission



Dec. 16, 1930. w- DUDLEY 1,784,825

SPEECH TRANSMISSION Filed Sept. 5, 1929 ATTORNEY a balancing network 18 therefor.'

of those portions of the circuit which are` Fig. 2 showsthe characteristics ofthe cur-l rent intensity distribution at variouspoints in the system; and

Figs. 3 and 4 illustrate different forms ofk equalizers suitable for use at the receiving end of thefsystem. y Y j In Fig. 1, T denotes a subscribers telephone set comprising' a microphone 10 and a `re` ceiver 11. The subscribers set is connected by means of a land line, denoted by 12, to the vhybrid coil 13 and balancingnetwork 14:, at

which point the circuit is divided into separate channels for transmission and reception respectively. The transmi ting channel comprises a band pass filter 15, the purpose of which is to exclude all speech frequencies save those necessary forv clear reception. Ther transmission range of this filter is preferably from abouttOO c. p. s. to 2500 c. p. s., the very strong low-frequencies not essential to clear articulation being thereby excluded, and also those high frequencies which would be too strongly attenuated in the cable to be utilized at the receiving end. Followingthe filter is a current intensity equalizer 16, and following this is a power amplifierfl'?. The

vfunction of the current equalizer is to bring all of the frequency components of normal speech tov about the same energy level before amplication. Its transmission characteristic is based on the normal frequency distribution of speech energy as described by Crandall, Sounds of Speech, Bell SystemA Technical Journal, Vol. IV,'No. 4:, October i 1925. Preferably the equalizer is made to 'favor the high frequency components slightly,

sincethese are of greatestimportance for good articulation. From theampliiier the transmission pathpasses to a second hybrid coil 18 to which is connected the cable 20andV Currents received from the cable enter the hybrid coil 18 and pass into the receiving circuit which comprises'in succession a current intensity equalizer 21, an amplifier 22, 'and transmission equalizer 23. From the transmission equalizer the currents passto.

hybrid coil 13 and thence to thejsubscribes set" T. vThe terminal apparatusat the other.

end of the cable is a duplicate ofthat shown the.. energy;l distribution represented' by in the figure and hence need not be repr0- duced. c

The relative values of the current intensities at different points in the circuit are shown by the curves of Fig. 2 in which the abscissae are proportional to frequency and the ordinatesto the logarithms of the energyi The relative values of the energies are plotted in terms 7 of the standard unit of transmission, the Decibel, and, since the scale is logarith'.-

I"mic,an arbitrary zero-is taken which `will be explained later. Curve a represents tlienor inal frequency distribution of speech energy and corresponds to the energy of transmitted currents at the input of equalizer 16. The [limits of the frequency range are represented by dotted lines g and g corresponding to the cut-off frequencies of filter 15. Curve b repl resents the distribution ofenergy at the output of amplifier 17, the change inthe shape of this curve from curve a being due to the characteristic of equalizer 1bl and its elevation in the energy scale being `due tothe am-v plifier, The shape of curves a and b will of course differ for dierent speakers, but, if

equalizer 16'is designed in accordance with*l tlieascertained average of a large number of voices, the differences will not be excessive and, by virtue of the upward slope given to curvewb,the high frequency components of most voices will be brought up to the desired"v level. Curve c represents the energy distri" `bution of thev currents at the receiving end of the cable or at ythe input ofreceiving equal# v,

izer 21. Curve d represents th'e"uniforlmly`,y

distributed thermal lagitation noise ofy the'j ycable itself, the level of this curve being l taken arbitrarily as zero since it represents a f value below which lthe level of the received4 currents Vmust not fall.` After passing I 'through the equalizer 21 .the/.received current j Adistributionis representedl by curvee. l Itis not necessary that 'complete equalization should be obtained at this point but only's'uch. degree as will enable all of `the frequencies to be handled by amplifier `22 withoutcover- ,Y

loading. Substantially complete equalization may'however be provided for if desired.'`

` Assuming that the equalizer 21 isnot'itself productive of thermal agitation noise, Athe noise levelatfthe amplifier is indicated by .I

curve f, Vcurves Ae and fdilfering from curves ac and d bythe samefamoun't so that theratio vof signal to noise is maintained., The effect) ofampliier22 and equalizer 23 Vis,to raise] Athe level of the receive'dfcurrents andto' rei store their` energy distribution to the normallimI represented by the curve yAt the same time thenoise currents representedgby. curve fare amplified vand an additional constant quantity' is addedy bythe smallshott effect in the amplifier, the :resultant noise currents being,

isa.

thereafter distorted-.by equalizer 23 to ,have

curve g. l.

CII

Suitable constructions for the equalizer 21 are illustrated in Figs. 3 and 4, the terminals 1, 2, 3 and 4, thereof corresponding to the corresponding terminals indicated in Fig. 1.

In Fig.V 3 the circuit comprises a series-shunt inductance combination comprising series inductances L;L and L2 and shunt inductance L3 connected therebetween. By the shunting acti-on of L3 the low frequencies are attenuated to a degree inversely proportional to the 3 capacities C1 and C2. The characteristic may be controlled to give any desired degree of discrimination by varying the inductances and the coupling of P and S and by varying the capacities C1 and C2.

No resistance elements are used in the equalizer 21 and, if the inductances and capacities are constructed with a view to avoiding energy dissipation, the equalizer will be substantially free from thermal agitation noise.

` It is to be noted that in the circuit described the currents in the cable input are substantially equalized in intensity, in consequence of which the currents at the output end have a relatively smooth distribution corresponding roughly to the attenuation characteristic of the cable. This simplifies the design of the equalizer 21 since only a continuous decrease of attenuation with frequency is required, such as can be obtained currents of perceptible magnitude.

l/Vhat is claimed is:

1. A telephone system comprising a submarine cable of trans-oceanic length, transmitting and receiving amplifiers therefor, and in combination therewith circuit means for equalizing the current intensities of normal speech at the input of the transmitting amplifier, circuit means for equalizing the infier for equalizing the transmission of the system as a whole. Y

2.y A telephone system in accordance with claim 1 in which the received current equalizer comprises only reactive impedance ele- .V

ments.

3. The method of receiving highly attenuated speech currents which comprises equalizing the intensities of the received currents of different frequencies at low energy level,

amplifying the equalizedcurrents uniformly and thereafter restoring the frequency distribution of the energy to normal.

4. A system for receiving telephone currents transmitted over a long submarine cable comprising in combination an amplifier, circuit means for equalizing the intensity of the received speech current components at the input of said amplifier and a transmission equalizer in the output of said amplifier forv restoring the energy distribution of the speech components to normal. Y

5. A system in accordance with claim 4 in which the input equalizer comprises only reactive impedance elements.

In witness whereof, I hereunto subscribe my name this 3rd day of September, 1929.

I-IOMER W. DUDLEY.

tensities of the received currents at the input of the receiving amplifier, and an equalizer in the output circuit of the receivlng ampli- 

